This invention relates generally to pressure vessels having adapter fittings mounted therein to provide access to the interior of the tank by plumbing connections. The invention is particularly concerned with techniques for manufacturing filament-wound plastic pressure vessels which are intended to contain corrosive and/or pressurized fluids. Pressure vessels of this type generally include a rotationally cast or blow-molded inner plastic liner which is wound with resin-impregnated glass filaments to provide adequate mechanical and structural properties for the side wall of the vessel. A typical pressure vessel would comprise a cylindrical side wall having oblate, ellipsoidal end portions, at least one of which is provided with an axial fitting which is adapted to be connected to a closure member or to plumbing connections.
According to the prior art, metallic and plastic fittings are associated with plastic tank liners by fusion or molding techniques, and the subassembly of the fitting and the liner are wound with resin-impregnated filaments. Examples of such pressure vessels may be found in U.S. Pat. Nos. 2,848,133; 3,874,544; 3,508,677; and 3,907,149.
The provision of a plastic fitting has some desirable aspects in that a thermoplastic fitting may be fused and cohered to a thermoplastic tank liner to provide a leakproof joint. Plastic fittings, however, tend to radially expand upon the application of pressure and temperature to permit leakage or failure relative to a fitting closure member or a plumbing connection. Such fittings are desirably reinforced by integrally molded filaments or chopped fibers, and the external tank windings must be geometrically arranged about the fitting to provide a radial restraint against expansion. Metal fittings, on the other hand, possess adequate mechanical properties, but are subject to pitting and corrosion if subjected to many chemicals. Moreover, relatively complex assembly techniques are required in positioning the fitting on the liner prior to the winding operation.
In U.S. Pat. No. 3,825,145, there is disclosed a container which is preferably formed by a rotational casting technique and which has access fittings molded into the side wall of the container during its manufacture. The fittings are mounted in position on the mold so that the fittings are at least partially embedded in the wall of the molded article after the rotational casting operation. Associating the fittings with the mold cavity, however, requires precise machining operations, and renders the mold suitable only for use in molding a container having a uniquely sized fitting.